Can You?

[onebto]

 

[SuperColey1]

as coley said, if a starter is underpowered lets say by 20% then that 54watt starter is kicking out aproximatly 43 watts, on a 39 watt tube, which is overpowered but not much, my theory maybe wrong but worth a crack

No you got it wrong. lol

What I mean is if you get a 15W ballast then you can use it to run a 20W tube BUT it will only put 15W into the 20W tube thus underpowering it. this is what was done on Fluval Uno and Duo hoods and is the way Arcadia do their starters. You can run 15-20W tubes but the actual ballast inside the box will be a 15W ballast and thus run the 20W tube at 15W, slightly underpowered.

If you were to try and run a 30W tube from the 15W ballast then it likely wouldn't have enough power to do anything but light the ends up.

If you were to run a 30W tube from a 50W ballast then it would probably work but would be highly likely to burn out rapidly.

Therefore you should always get equal or close to the W of tube quite simply so that it either lights wit full wattage or is slightly underpowered.

Just get a cheap tube from a hardware store and try it. Not over your tank and leave it on for 8 hours. See what happens.

If it works then take the risk with a decent tube.

The ballast's function is to control the electrical current running through the glass bulb so that there is just enough current flowing through so that the light ignites and is maintained, but not so much that the bulb is destroyed.
AC

 

[Truck]

ahh, i get it now

 

[ROSSCO22]

Well Ive sorted the Ebay issue with the 54 watt one and I am going to buy a 39watt unit.

 

[onebto]

 

[Night Crawler]

Run 39watt T5 tubes on a 54watt ballast?Or will it blow the tubes?

Simple answer... Yes you can.
WHY?
Because...
The wattage rating is the amount of power the ballast can push out safely. So for example... A 100W ballast will haply push out 50W, but if you try to draw out 150W then it cant cope and will overheat or just blow.
Another example. Try to push a car on your own. Its hard work. Try to push a car with a friend and its easy.

As for the mobile phone thing before. You were on about voltage which is completly different to wattage and is not relevent in this subject.
An example here would be your computer that you are sitting in front of (or beside). The USB ports on your computer are 5 Volts but only 500mA (I think) But the power supply in the back of your computer, which also provides 5 Volts has a much higher output 150W or more.

The voltage of a T5 lamp will usually be the same across all power outputs unles they specifically say otherwise. Thats why you can put a normal house/shop tube into lets say... An arcadia light unit.

 

[Schmill]

From what I understand of the ballasts and tubes, and contradicting Night Crawler above, no, you can't (read SHOULDN'T) mismatch your ballast and your tube. To know why you have to understand a little about how they work, and I believe that it's this that has thrown NC above.

In a standard lightbulb (incandescent lamp) you apply a voltage across the filament. At this point the filament has a low resistance so lots of current flow. The filament heats up (and glows giving out light), and as it heats it's resistance increases, reducing it's current draw. At a point that appears almost instant to anyone watching the resistance / heat / current of the filament even out and the bulb self regulates. (If it cools slightly it's resistance drops slightly, causing it to draw slightly more current, heating it up slightly, and everything is back in check. Similarly if it starts to get too hot, then it's resistance will increase, decreasing the current drawn, thus cooling the filament slightly and again everything is back in check)

Flourescent tubes do NOT work this way.
A flourescent requires a massive voltage to 'spark' the tube, that is to start the tube conducting. However, as a flourescent tube increases in temperature it's resistance REDUCES. A lower resistance results in more current drawn, which in turn results in more heating, lower resistance, more current, more heating.... This is classed as 'thermal runaway' and is common to various electronics as well as these tubes.
Connect a flourescent tube to an 'unlimited current' supply, and it will burn itself out.
This is where the ballasts come in. A ballast rated at 25-30W means that it will limit the current output in such a manner that the tube will not be able to draw more current than is required to run the lamp at approx. 25W. It also means that the ballast is capable of producing the initial 'spark' for the tube to ignite.

From this you can hopefully see that a ballast has both an upper and lower limit. It must be able to provide a sufficient 'spark' to ignite the tube in the first place, but it must also limit to current feeding the tube to achieve the correct wattage.

Some examples;

- Within Limits:
25-30W Ballast
25W Tube
Ballast can 'spark' the tube, and will run the tube at 25W

- Within Limits:
25-30W Ballast
30W Tube
Ballast can still 'spark' the tube, and will run the tube at 25W

- Oversized Tube:
25-30W Ballast
38W Tube
Ballast is likely to not be able to 'spark' the tube, if it does it may take a few attempts which reduces the life of the ballast. (Whether it can or not will depend on how much 'overhead' the manufacturer has on their ballasts, and also the age of the ballast)
If it DOES manage to spark the tube, and keep it lit, it will still be capping the current as for a 25W tube, so the tube will appear dim.

- Oversized Ballast:
25-30W Ballast
15W Tube
Ballast will 'spark' the tube, but will only cap the current flow to give 25W. This means you will be running your 15W tube at 25W. It may start out brighter, but will soon darken as the phosphorous is burnt away before the tube finally give out. (That is if the tube doesn't physically break first)

Hopefully that makes sense